1. Field of the Invention
The present invention relates to an exposing device that irradiates scanning light onto the surface of an image carrier and an image forming apparatus incorporating the same.
2. Description of the Related Art
In an image forming apparatus employing an electrophotographic process, the exposing device irradiates scanning light onto the surface of a pre-charged image carrier, for example, a photoconductive drum. Accordingly, an electrostatic latent image is formed on the surface of the drum and a toner image corresponding to the electrostatic latent image is transferred and fixed onto a sheet of paper.
A higher degree of accuracy is required when installing the exposing device in the image forming apparatus main body. This is because only a slightest displacement of the irradiation position of scanning light on the photoconductive drum can have a direct influence on the image quality. In view of the foregoing, a technique for an exposing device that takes into account a temperature change inside the apparatus main body has been disclosed (for example, see JP-A-2000-180766).
In the prior art above, a unit main body incorporating optical devices, such as a polygon mirror and a reflection mirror, is fixed to the apparatus main body at a portion where the rigidity is relatively low. More specifically, a unit main body of an almost square shape when viewed in plane is fixed to the apparatus main body at the center of the respective edges apart from the corners. The purpose of this configuration is to lessen an amount of deformation by making the rigidity homogeneous across the entire unit main body.
However, in a case where all the four edges are immovable as with this unit main body, the unit main body is not able to respond to a temperature change. The inability is attributed to a phenomenon that when the temperature of the unit main body rises, heat stress develops at least toward the center portion of the unit main body. This phenomenon becomes particularly noticeable when a polygon mirror is incorporated therein. The relative positions between the reflection mirror and the drum vary with the level-rising or -falling at the center portion of the unit main body, which poses a problem that a desired irradiation position cannot be attained. The technique in the prior art therefore has a still-unsolved problem regarding a response to a temperature change.